A fluorinated phenylenediamines such as tetrafluoro-m-phenylenediamine is an important intermediate for the synthesis of pharmaceutical preparations, pesticides, and macromolecular compounds, and is particularly useful as a raw material for a fluorine-containing polyimide having a low dielectric constant and a low refractive index, absorbing light sparingly, and having excellent water repellency.
Tetrafluoro-m-phenylenediamine, for example, has been heretofore produced by the ammonolysis of hexafluorobenzene or pentafluoroaniline or by a process of reacting pentafluoroaniline with such a nucleophilic reagent as phthalic imide which can add an amino group thereby inducing the substitution of an amino group for the fluorine atom and then converting the fluorine atom into the amino group. For example, Example 3 of U.S. Pat. No. 3,461,135 discloses a method for producing tetrafluoro-m-phenylenediamine by reacting pentafluoroaniline with phthalimide potassium and redistilled dimethyl formamide, adding ethanol and hydrazine to the mixture, refluxing the resultant reaction solution, and subsequently adjusting the pH of the refluxed product with sodium hydroxide to a weak alkali. This method, however, produces tetrafluoro-m-phenylenediamine in such a low yield as 13.4% and, by-produces tetrafluoro-p-phenylenediamine which is an isomer thereof as well as the tetrafluoro-m-phenylenediamine. As mentioned above, when tetrafluoro-m-phenylenediamine is to be produced, tetrafluoro-p-phenylenediamine is generally by-produced simultaneously at an approximate ratio in the range of 8: 2 to 9:1.
The tetrafluoro-m-phenylenediamine aimed at, therefore, must be separated and purified as by a process of distillation, recrystallization, column chromatography, and sublimation, for example. It is, however, extremely difficult to separate and purify the tetrafluoro-m-phenylenediamine and the by-produced tetrafluoro-p-phenylenediamine from each other.
JP-B-47-6,294 discloses a method for separating and purifying tetrafluoro-m-phenylenediamine, which comprises acylating crude tetrafluoro-m-phenylenediamine containing tetrafluoro-p-phenylenediamine thereby precipitating the m-isomer in the form and subjecting the residual filtrate further to diacylation and hydrolysis, and separating the m-isomer in the filtrate in the diacylated form. This method also requires to involve a process of separation and purification or tetrafluoro-m-phenylenediamine.
Another known method for producing tetrafluoro-m-phenylenediamine comprises reacting tetrachloroisophthalonitrile in a benzonitrile medium with a fluorinating agent at a temperature in the range of 190-400° C. under spontaneous pressure (JP-B-63-5,023) thereby producing tetrafluoroisophthalonitrile, transforming this product to the form of a diamide (tetrafluoroisophthalamide), and subjecting the resultant tetrafluoroisophthalamide to Hoffmann rearrangement (“Synthesis of fluoride compound and function,” pp. 204-205, published by CMC K.K. on May 6, 1987).
Yet another known method for producing tetrafluoro-m-phenylenediamine comprises reacting tetrafluoroisophthalic acid with sodium azide in a strong acid thereby producing tetrafluoro-m-phenylenediamine (JP-A-2001-226,329). In Example 2 thereof, tetrafluoro-m-phenylenediamine is produced in a high yield of 74.0%. In Comparative Example 1 of this official gazette, a method for obtaining tetrafluoro-m-phenylenediamine which comprises using tetrafluoroisophthalamide as a raw material, adding sodium hydroxide and bromide thereto thereby converting the group, —CONH2, into a group, —CONHBr, extracting the resultant compound with isopropyl alcohol, and hydrolyzing the extract by the addition of hydrochloric acid is described. This method comprises causing a mixed solution of sodium hydroxide and bromine to act on the amide moiety of tetrafluoroisophthalamide thereby converting the amide into an amine by Hoffmann rearrangement and eventually obtaining tetrafluoro-m-phenylenediamine. The yield of this product is 13.8%.
Since the Hoffmann rearrangement which is effected in the method described above uses a strong alkali such as sodium hydroxide in an excess amount, however, the fluorine atoms are partially converted into a hydroxyl group. On other words, as described in detail in Comparative Example 1, the Hoffman rearrangement in a strong acid entails the problem of lowering the yield of tetrafluoro-m-phenylenediamine as the target product.